Hi Nelson,
I spoke about this in the news post that I posted yesterday: https://pitwall.org/news/76/scouting-and-person-discovery-changes
The blue stars got their introduction in a recent update. They are used to indicate how good the potential and current rating of persons are. If the "Known" statistic is not at the maximum level, then the stars will be blue. The known statistic can only increase, and increases by competing in races for drivers, or by being assigned to a position for staff. There are some small other parameters that play an influence to that, but that's the general idea.
So basically, there is the known stat: we can't influence that. It is calculated by the game. A person gradually becomes known if he is assigned to a position in the game. Certain positions have a higher multiplier than others, so for example a junior designer will need more weeks to become fully known, than a division 1 driver.
As long as the known statistic is not maxed out, blue stars for potential and current ability are displayed. While they are blue stars, you are not certain about the accuracy of the stars. Since yesterdays update it can be either rated too high, too low, or could be quite accurate.
The stars themselves are always set at a relative level. It only takes persons into account who are known in the game. There are for example 20/20 potential designers out there, but since they're not known they're not taken into account. Therefore, a 15/20 potential designer could be 5 stars now, but could become a 4 star later when a 20/20 is discovered.
For youth (-18) the stars are calculated to persons from the same age level. Once they become adults (18+) the maximum is looked up for the entire adult population.